This invention relates to the art of field emission displays (FED) and, more specifically, to a process and apparatus for improved grey-scale control in FED devices.
FIG. 1 is a cross-sectional view of a typical field-emission display. As seen, the FED comprises a faceplate 16, with a luminescent phosphor coating placed thereon, separated from a backplate, or substrate 11. On the backplate 11, there is formed a plurality of emitters 13 which are electrically connected at the base to a column electrode 12. Near the tip of the emitters there is provided an extraction grid 15. In order to generate a display on the faceplate 16, a voltage differential 20 is provided between the extraction grid 15 and the column electrode 12. This causes electron emission, often referred to "Fowler-Nordheim" emission, from the emitter tips. The electrons 17 are drawn to the faceplate 16 by electrostatic attraction, where they strike the phosphor coating causing illumination of the faceplate 16. In practice, the substrate of the FED is subdivided into a plurality of independently addressable pixels 22, each pixel having an array of emitters for causing illumination. A more detailed description of general FED technology is found in "Field-Emission Displays" by David A. Cathey, Jr., incorporated herein by reference.
One problem encountered in the manufacture of FED's is that a linear increase in the extraction grid voltage results in an exponential increase for the emitter current. This makes control of the illumination level of a given pixel difficult, and increases the odds of an undesirable effect, such as a pixel failure. In order to prevent these undesirable effects, a resistor or resistive layer is often formed between the emitter tips and the emitter conductor, as described in U.S. Pat. No. 4,940,916 to Borel, and U.S. Pat. No. 4,387,844 to Browning, both incorporated herein by reference.
With a current limiting resistor in series with the emitter tip, the current increases initially as a power function, described by the FN equation; but as the current becomes large enough, the linear current/voltage characteristics of the resistor begins to dominate, and the current increase becomes approximately linear for linear increases in grid voltage. With a large current limiting resistor, for example, one or more gigaohms, the resistor can stabilize the tip emission even at very low current levels, for example, in the nanoamp range. This occurs because excursions in the emitter current are limited by the resistor generated voltage drop, so as the current increases, the tip to grid voltage decreases and emission then decreases. This provides good control of low luminance grey levels. However, the large resistance also requires a large excursion of the grid voltage in order to achieve high luminance levels which require larger currents.
With a relatively small current limiting resistor, for example, 100 megohm, higher luminance level currents can be achieved with a much smaller range of grid voltages, but the low gray levels receive very little stabilization benefit; hence, the power function dominates the emission. This is because the voltage drop across the low value resistor causes insignificant changes in the tip to grid voltage during current excursions. For example, a 1 nanoamp current would cause a 1 volt drop across a 1 gigaohm resistor but only a 0.1 volt drop across a 100 megohm resistor. While a one volt drop is significant, a 0.1 volt drop would have little effect.
In a passive matrix FED, the display pixel is typically addressed by row and column lines, or electrodes. The column lines are typically used for gray scale control. The column line is electrically connected to the emitter tips of a pixel by a resistor. This resistor may be a film or layer having the desired resistivity. This layer provides the current limiting resistance for stabilizing the emission.
The row line of the display is connected to the emitter grid. Therefore, a display pixel is addressed by driving the grid (row) positive with respect to the ground, and by driving the emitter tip (column) negative. Alternately, the tip is biased positively normally, and then pulled toward ground to address the pixel. This is the scheme that will be used herein for purposes of illustration. However, those with skill in the art will recognize that other addressing schemes could be used.
In a passive matrix FED, it is desirable to reduce the required grid voltage range to conserve power in the drive circuitry. But, it is a also important to maintain stable emissions at low luminance gray levels to avoid problems with flickering tips, tip-to-tip, non-uniformities and luminance variations across the display. Therefore, there is a need in the art for an apparatus which will overcome the above mentioned difficulties.